One thing that amazes me about astronomy is that even after, what, 30 years of doing it (I was really young when I started) I still get surprised at some basic facts.

That picture above is the spiral galaxy M101, a staple of amateur observing. It’s a big, bright, face-on spiral, and since it’s in Ursa Major (near the Big Dipper) it’s up most of the year. I’ve seen it a few times myself, though usually when I’m looking in books. Actually, in a lot of those books, it’s claimed that our own Galaxy would look like M101 if you could get outside of it.

But boy, is that ever wrong! M101 is a lot bigger than the Milky Way. A lot. It’s 170,000 light years across, compared to 100,000 for us. We have an impressive 100 billion stars in the Milky Way, but M101 is bursting with something like a trillion stars, ten times the number in the Milky Way! That’s staggering. I had no clue M101 was so huge.

The picture above is a stunning new release from Hubble (click it for a higher-res version). It’s almost as awesome as the galaxy itself: the original image is 12,000 x 16,000 pixels in size, making it one of the largest images of a spiral galaxy ever produced. The images span nearly a decade and were originally taken for several different projects (it was nice to see the name of my old friend Kip Kuntz as the head of one of those projects; he assisted me way back when on my very first trip to Baltimore to work on Hubble data for my PhD). It took 54 separate Hubble pointing to span the face of this galaxy.

But even the medium res image has delights. For example, the image on the left, sliced out of the piddling 10Mb image (on the tip of the spiral arm on the extreme left in the image), shows one of 3000 star clusters found in M101. Take a look at it: each blue dot is actually a massive giant star, much hotter and bigger than the Sun, and each of which is doomed to explode as a supernova someday. There must be many billions of such stars in M101. Some of them are packed so tightly together in that image they’re just a blur. Imagine what the sky must look like from a planet near there…

And the image on the right is a background galaxy (just above and to the left of the center of M101) that is probably something like 20 or more times farther away than M101’s 25 million light year distance. Spirals are generally blue, but no doubt this one is significantly reddened by dust in M101, like a sunset is reddened by dust suspended in air.

Ironically, that galaxy is probably more like our Milky Way: it has a rectangular bar across the center, just like our Galaxy does. A bar like that is due to complicated gravitational interactions in the centers of galaxies, and for some reason M101 doesn’t have one. That’s interesting all by itself.

I could go on and on. The image is incredibly rich in detail! So find yourself a resolution you’re comfortable with, sit back, and skim across the surface of this magnificent galaxy. With a trillion stars, and billions of solar masses of gas and dust, there’s enough there for everyone to enjoy.

Absolutely fantastic photos! Just one nit to pick: while it may be dust that reddens the galaxies we see behind M101, that’s not the only thing which colors the sunsets here on Old Terra. The reason the sunsets (or “sunclipses”, as Buckminster Fuller’s family would say, to be less geocentrist) are orange, yellow and red is all tied up with the reason the daytime sky is blue. To quote the Physics FAQ,

Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Even today, people sometimes incorrectly say that this is the case. Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment. He was even able to use the calculation as a further verification of Avogadro’s number when compared with observation. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

Dust can make a sunset redder, if there’s enough of it, but it’s not the sole thing responsible for the color:

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away. If the air is polluted with small particles, natural or otherwise, the sunset will be more red. Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers. The sky around the sun is seen reddened, as well as the light coming directly from the sun. This is because all light is scattered relatively well through small angles–but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours.

Surprisingly, I have had a telescope for 7 years, and have never managed to see this galaxy. Partly because it’s hard to see from where I am (north-ish looks over the city and the resulting city-glow), and partly because I’m still a bit of a doofus about aligning my telescope properly

I have sought help for the latter though. I found some local astronomers who have volunteered to give me the benefit of their experience so I can use my telescope more effectively.

Question: I have 80mm and 100mm binoculars. Is M-100 within reach of those? I’ve never really tried, but they’re easier to use than the telescope and tend to be better for viewing wide objects.

Hey Squid, the answer is yes, you should be able to see M101 with binos. However: you will probably need to get out under a dark sky somewhere, and you will probably only see the bright core. But it’s still pretty damned cool to feel those photons finish their 25 million light year journey on your retina.

Well, I was insane to download the big thing. I’ve been looking at it, lot’s of stuf there. Can anyone tell me what is the green thing near the middle of the lower left panel in this image I build from it?

And as you’re gazing at this incredible picture, imagine that somewhere among the trillion stars there are planetary systems with intelligent life (does ours qualify?) with “people” gazing through their telescopes at the Milky Way pondering the same questions as we are.